[RFC][PATCH] ipc: Remove IPCMNI

[Matthew Wilcox]

On Wed, Mar 14, 2018 at 07:49:29PM -0500, Eric W. Biederman wrote:
> @@ -109,20 +109,13 @@ void free_ipcs(struct ipc_namespace *ns, struct ipc_ids *ids,
>  {
>  	struct kern_ipc_perm *perm;
>  	int next_id;
> -	int total, in_use;
>  
>  	down_write(&ids->rwsem);
> -
> -	in_use = ids->in_use;
> -
> -	for (total = 0, next_id = 0; total < in_use; next_id++) {
> -		perm = idr_find(&ids->ipcs_idr, next_id);
> -		if (perm == NULL)
> -			continue;
> +	next_id = 0;
> +	while ((perm = idr_get_next(&ids->ipcs_idr, &next_id))) {
>  		rcu_read_lock();
>  		ipc_lock_object(perm);
>  		free(ns, perm);
> -		total++;
>  	}
>  	up_write(&ids->rwsem);
>  }

We have a helper for this:

	idr_for_each_entry(&ids->ipcs_idr, perm, next_id) {
		rcu_read_lock();
		ipc_lock_object(perm);
		free(ns, perm);
	}

(using idr_get_next() is tricky because you have to remember to increment
next_id yourself, and you didn't).

> +static int ipc_idr_alloc(struct ipc_ids *ids, struct kern_ipc_perm *new)
>  {
> +#ifdef CONFIG_CHECKPOINT_RESTORE
> +	if (ids->next_id >= 0) {
> +		idr_set_cursor(&ids->ipcs_idr, ids->next_id);
>  		ids->next_id = -1;
>  	}
> +#endif
> +	return idr_alloc_cyclic(&ids->ipcs_idr, (new), 0, 0, GFP_NOWAIT);
>  }
>  
> -#endif /* CONFIG_CHECKPOINT_RESTORE */

That seems a little convoluted; is there a reason to not call idr_set_cursor()
instead of assigning to ids->next_id?

> @@ -757,30 +725,20 @@ static struct kern_ipc_perm *sysvipc_find_ipc(struct ipc_ids *ids, loff_t pos,
>  					      loff_t *new_pos)
>  {
>  	struct kern_ipc_perm *ipc;
> +	int id;
>  
> +	/* Out of range - return NULL to terminate iteration */
> +	if (pos > INT_MAX)
>  		return NULL;
>  
> +	ipc = idr_get_next(&ids->ipcs_idr, &id);
> +	if (!ipc)
> +		return NULL;
>  
> +	*new_pos = id + 1;
> +	rcu_read_lock();
> +	ipc_lock_object(ipc);
> +	return ipc;
>  }

I'm no expert on the IPC locking, but I would have thought you'd want to
call rcu_read_lock() before calling idr_get_next() to avoid a simultaneous
delete from freeing 'ipc'.

Oh, I see.  proc_start takes the rwsem for read and proc_stop releases it.
The locking here seems quite shabby and in need of renovation.